By combustion of fuel in steam generators and combustion equipment having a fluidized bed furnace, the size of fuel particles must be such as to enable, at the lowest thermal or steam output of the equipment, that is at the smallest flow rate of combustion air, fluidization of all particles both of fuel and solid residues from the combustion process. In addition, the fuel must be granular to enable its delivery at multiple points of the fluidized bed. It is also usually assumed that there must be at least one fuel inlet per square meter of the fluidized bed area. Fluidized bed combustors use particles coarser by at least two orders of magnitude in comparison with particle sizes used at present with steam generators for combustion of powdery fuels. Thus, it is not possible to use standard types of crushers and classifiers. Therefore, simultaneously with the development of steam generators with fluidized bed combustors the development of equipment for treatment of fuel must take place.
In equipment used at present for treatment of fuel for combustion, the fuel is dried by hot exhaust gases, or by hot air by steam. In drying of fuels by exhaust gases, the gases are sucked in by turbo-blowers and are compressed to the pressure needed for classification and transport of coal dust, while particles of fuel are picked up by exhaust gases simultaneously from the crushers and passed to classifiers from which the coarse particles return back in the crusher. From the classifiers the fuel is conveyed together with secondary air into the combustor. (Perry J. H.--Chemical Eng. Handbook 9-19 and next pages, McGraw Hill 1963). The steam dryers are seldom used and are situated ahead of the crusher in which the fuel is again finally dried by hot gases. Far drying of the fuel by hot air, the arrangement of the equipment is similar to that for drying of fuels by hot exhaust gases. The purpose of the invention is to present a simpler and more effective method and apparatus for treatment of fuels by drying and classification by use of waste heat of combustion in fluidized bed furnaces.